We travelled through the Red Centre and up to Kakadu/Litchfield last year and never had any issues at all. I have never had anything stolen from me at a campsite - I don't know if this is the norm or not.
I have read previously that you close up your camper zips, leave a table and chairs out so it looks like you are going to be back any minute, and don't leave things like ipods, laptops laying around, so we have done that when we have gone out.
In saying this, if ever I did see someone rummaging through my stuff, they had better pray that they can run faster than I as I believe that stealing is the lowest of the low and I would break every finger on each hand to permanently remind them not to do it again.